‘ ‘ Is Denser Greener ? An Evaluation of Higher Density Development as an Urban

Jacob and Lopez (2009) presents modeling results to support the conclusion that the per capita water quality impacts of urban development decrease significantly with population density. If true, this conclusion would have important implications for land use planning throughout the world. However, as discussed below, the modeling is based on questionable assumptions that likely bias the conclusion. Water quality in this paper is defined in the context of total phosphorus, total nitrogen, and total suspended sediment load. The annual loading of each of these pollutants is estimated as the product of annual runoff and a pollutant concentration. The runoff amount is computed as the product of annual precipitation and a runoff coefficient that depends solely on impervious fraction, which in turn is assumed to depend on the number of dwelling units per area. The pollutant concentration is computed as a sole function of impervious fraction, based on regression equations from a previous study of small watersheds in the Austin, Texas area. Application of the model indicates that for all three pollutants, the annual loading per dwelling unit decreases with increasing development density. This follows directly from the sole focus on impervious surfaces, which decrease in area per dwelling unit with increasing development density, while pollutant concentrations are assumed to remain constant or increase slightly. I have several concerns about the assumptions used to support the conclusions of this paper. By focusing exclusively on sediment and nutrient loads, the paper fails to account for other important water quality impacts that are highly sensitive to development density. The modeling does not account for the fact that the mitigation of water quality impacts depends strongly on development density, nor does it acknowledge the fact that the magnitude of water quality impacts depend on their spatial distribution. Lastly, the paper does not account for the fact that urban and suburban development of agricultural lands can result in a reduction in phosphorus, nitrogen, and sediment loads.